In many systems utilizing electronic signals, an unwanted resonant spike or other large-magnitude narrow band signal is present in an otherwise useful signal due to physical properties of the system. One example of such a system is a control system for adjusting the mirror segments of a segmented mirror, such as is used in astronomy or light beam steering applications. These mirror segments often have a resonant frequency which typically introduces a large phase shift in the control system feedback loop which can destabilize the system. Unless the system bandwidth is much lower than the mirror resonant frequency, an appropriate phase margin required for system stability is difficult to achieve.
The traditional method of removing or suppressing such a narrow band signal is through the use of a narrow band stop or notch filter centered around the center frequency of the narrow band signal. However, the center frequency of the narrow band signal may often vary in both frequency and amplitude as a function of temperature, loop band-width, or other system parameters. Such variation imposes a severe restriction on a fixed frequency notch filter. Obtaining sufficient attenuation over the variation in the narrow band center frequency has required the use of a low Q filter or multiple filters. In addition to being less efficient, each of the low Q filter and the multiple filter approaches, when used in a feedback amplifier, can add a significant phase lag at the unity gain crossover frequency of the system. The corresponding reduction in phase margin results in a reduced bandwidth or lower relative stability. More desirable would be a high Q notch filter with automatic frequency tracking to suppress a narrow band signal while tracking it through frequency variations.